Recessed access device

ABSTRACT

A method of manufacturing a recessed access device includes the following operations. A first trench is formed in a substrate. A first gate oxide layer is formed on an inner surface of the first trench. A sacrificial layer is formed in a bottom of the first trench, in which a portion of the first gate oxide layer above the sacrificial layer is exposed from the first trench. The portion of the first gate oxide layer is removed to expose a sidewall of the first trench. The sidewall of the first trench is oxidized to form a second gate oxide layer within the substrate, in which the second gate oxide layer is in contact with the first gate oxide layer. The sacrificial layer is removed to form a second trench.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a Divisional application of U.S. applicationSer. No. 17/167,067, filed Feb. 3, 2021, which is herein incorporated byreference in their entirety.

BACKGROUND Field of Invention

The present invention relates to a recessed access device.

Description of Related Art

With the development of semiconductor technology, the dimensions ofsemiconductor devices have been shrinking. The techniques of highdensity of transistors and transistors with shorter channel lengths arehighly demanded to increase the operating speed and the performance ofthe devices. However, the shorter channels lengths have the problem ofshort channel effects, which limit the performance of the devices.

Metal gate recessed access devices (RADs) for dynamic random accessmemory (DRAM) have been demonstrated to avoid short channel effects.However, gate induced drain leakage (GIDL) may occur between the gatelayer and the source/drain region in the semiconductor devices.Therefore, there is a need for a novel recessed access device techniqueto overcome the problem mentioned above.

SUMMARY

One aspect of the present disclosure is to provide a recessed accessdevice. The recessed access device includes a substrate, a trench, agate oxide layer, a gate layer, and a source/drain region. The trench isembedded in the substrate. The trench includes an upper portion and alower portion, the upper portion has a substantially vertical sidewalland a first curved sidewall extending from a bottom edge of thesubstantially vertical sidewall to a top edge of the lower portion, andthe first curved sidewall of the upper portion and a sidewall of thelower portion form an obtuse angle. The gate oxide layer is disposed inthe trench. The gate oxide layer has a first portion on thesubstantially vertical sidewall, a second portion on the sidewall of thelower portion, and a connection portion on the first curved sidewall andbetween the first portion and the second portion. A maximum thickness ofthe connection portion is greater than a first thickness of the firstportion and a second thickness of the second portion. The gate layer isdisposed in a bottom of the trench to cover the second portion and theconnection portion of the gate oxide layer. The source/drain region isdisposed in the substrate and adjacent to the gate layer.

According to some embodiments of the present disclosure, the firstthickness of the first portion is substantially the same as the secondthickness of the second portion.

According to some embodiments of the present disclosure, the firstthickness of the first portion is different from the second thickness ofthe second portion.

According to some embodiments of the present disclosure, the connectionportion has a top surface substantially coplanar with a top surface ofthe gate layer.

According to some embodiments of the present disclosure, the connectionportion includes a second curved sidewall facing toward the gate layer,and the second curved sidewall extends from a top edge of the gate layerto a top edge of the second portion of the gate oxide layer.

According to some embodiments of the present disclosure, thesource/drain region is adjacent to the connection portion, the firstportion, and a portion of the second portion of the gate oxide layer.

According to some embodiments of the present disclosure, the recessedaccess device further includes a dielectric layer disposed on the gatelayer.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a flowchart of a method of manufacturing a recessed accessdevice in accordance with some embodiments of the present disclosure.

FIG. 2A to FIG. 2J are cross-sectional views of a method ofmanufacturing a recessed access device at various stages in accordancewith some embodiments of the present disclosure.

FIG. 2K is a cross-sectional view of a recessed access device inaccordance with some embodiments of the present disclosure.

FIG. 3 is a flowchart of a method of manufacturing a recessed accessdevice in accordance with some embodiments of the present disclosure.

FIG. 4A to FIG. 4H are cross-sectional views of a method ofmanufacturing a recessed access device at various stages in accordancewith some embodiments of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed. Itshould be understood that the number of any elements/components ismerely for illustration, and it does not intend to limit the presentdisclosure.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of the embodiments. As usedherein, the term “and/or” includes any and all combinations of one ormore of the associated listed items.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

In metal gate recessed access devices (RADs), gate induced drain leakage(GIDL) is one of the most important issues on the memory refreshperformance of a memory device. GIDL may occur in the overlap regionbetween a gate layer and a source/drain region in the semiconductordevice, and this is because of a higher electric field under deviceoperation. GIDL may be avoided by manufacturing a RAD structure with athicker gate oxide layer to separate the gate layer from thesource/drain. Therefore, the leakage current could be avoided. However,a cross-section of the gate layer would also be reduced by the thickergate oxide layer, thereby reducing a drive current of the device.Therefore, the thicker gate oxide layer could avoid GIDL, but theability to control the gate layer would be worse because of thereduction of the drive current of the device.

The present disclosure provides a recessed access device which could notonly improve the GIDL problem as mentioned above, but also not to affectthe ability to control the gate layer of the recessed access device. Thepresent disclosure also provides methods of manufacturing the recessedaccess device. Embodiments of methods of manufacturing the recessedaccess device will be described in detail below.

FIG. 1 is a flowchart of a method 100 of manufacturing a recessed accessdevice in accordance with some embodiments of the present disclosure.Operation 110, a first trench is formed in a substrate. The methodcontinues with operation 120 in which a first gate oxide layer is formedon an inner surface of the first trench. Operation 130, a sacrificiallayer is formed in a bottom of the first trench, in which a portion ofthe first gate oxide layer above the sacrificial layer is exposed fromthe first trench. The method continues with operation 140 in which theportion of the first gate oxide layer is removed to expose a sidewall ofthe first trench. Operation 150, the sidewall of the first trench isoxidized to form a second gate oxide layer within the substrate, inwhich the second gate oxide layer is in contact with the first gateoxide layer. The method continues with operation 160 in which thesacrificial layer is removed to form a second trench. Operation 170, agate layer is formed in a bottom of the second trench. The methodcontinues with operation 180 in which a source/drain region is formed inthe substrate and adjacent to the gate layer.

FIG. 2A to FIG. 2J are cross-sectional views of a method ofmanufacturing a recessed access device 2000 at various stages inaccordance with some embodiments of the present disclosure.

Please refer to FIG. 2A, a first trench 220 is formed in a substrate 210(the operation 110 of FIG. 1 ). In some embodiments, the substrate 210may be, for example, a bulk single crystal silicon substrate, asilicon-on-insulator (SOI) substrate, or some other suitablesemiconductor substrate. In some embodiments, the substrate 210 is madeof silicon (Si), germanium (Ge), silicon carbide (SiC), gallium arsenide(GaAs), and/or a combination thereof or other suitable semiconductorsubstrates.

Please refer to FIG. 2A and FIG. 2B, a first gate oxide layer 240 isformed on an inner surface 230 of the first trench 220 (the operation120 of FIG. 1 ). Specifically, the first gate oxide layer 240 has a topportion 240 a and a bottom portion 240 b, and the top portion 240 a isconformally formed on a sidewall 232 of the first trench 220, and thebottom portion 240 b is conformally formed in a bottom 234 of the firsttrench 220. In some embodiments, the material of the first gate oxidelayer 240 may be silicon oxide, such as tetraethylorthosilicate (TEOS),silicon dioxide (SiO₂), or silicon oxynitride (SiON), or combinationsthereof, but not limited thereto.

Still referring to FIG. 2B, in some embodiments, the first gate oxidelayer 240 is performed by a deposition process. In some embodiments, thefirst gate oxide layers 240 can be formed by deposition methods such aschemical vapor deposition (CVD), atomic layer deposition (ALD), physicalvapor deposition (PVD), or other suitable deposition processes. Inalternative embodiments, the first gate oxide layer 240 is formed byperforming a thermal oxidation process to oxidize the inner surface 230of the first trench 220. More specifically, the first gate oxide layer240 is thermally grown on the inner surface 230 and in the substrate210.

Please refer to FIG. 2B to FIG. 2D, a bottom portion 250 b of asacrificial layer 250 is formed in the bottom 234 of the first trench220, in which the top portion 240 a of the first gate oxide layer 240above the bottom portion 250 b of the sacrificial layer 250 is exposedfrom the first trench 220 (the operation 130 of FIG. 1 ). As shown inFIG. 2B and FIG. 2C, the sacrificial layer 250 is formed on the firstgate oxide layer 240. In detail, the sacrificial layer 250 has a topportion 250 a and a bottom portion 250 b. Specifically, the top portion250 a is formed on a sidewall 242 of the top portion 240 a of the firstgate oxide layer 240, and the bottom portion 250 b is formed on thebottom portion 240 b of the first gate oxide layer 240 in the bottom 234of the first trench 220. In some embodiments, the material of thesacrificial layer 250 may be nitride, such as silicon nitride, but notlimited thereto. In some embodiments, the sacrificial layer 250 isformed by deposition methods such as chemical vapor deposition (CVD),atomic layer deposition (ALD), physical vapor deposition (PVD), or othersuitable deposition processes. In some embodiments, the material of thefirst gate oxide layer 240 is different from the material of thesacrificial layer 250.

Still referring to FIG. 2C and FIG. 2D, the top portion 250 a of thesacrificial layer 250 is removed, and the sidewall 242 of the first gateoxide layer 240 above the bottom portion 250 b of the sacrificial layer250 is exposed from the first trench 220. In some embodiments, the topportion 250 a is removed by an etch back process, such as wet etching.

Please refer to FIG. 2D and FIG. 2E, the top portion 240 a of the firstgate oxide layer 240 is removed to expose the sidewall 232 of the firsttrench 220 (the operation 140 of FIG. 1 ). In detail, a top surface 244of the bottom portion 240 b of the first gate oxide layer 240 is alsoexposed. In some embodiments, the top portion 240 a is removed by anetch back process, such as wet etching.

Please refer to FIG. 2E and FIG. 2F, the sidewall 232 of the firsttrench 220 is oxidized to form a second gate oxide layer 260 within thesubstrate 210, in which the second gate oxide layer 260 is in contactwith the bottom portion 240 b of the first gate oxide layer 240 (theoperation 150 of FIG. 1 ). In detail, a top edge 246 of the bottomportion 240 b of the first gate oxide layer 240 is in contact with andembedded in the second gate oxide layer 260. Specifically, the secondgate oxide layer 260 is formed by a thermal oxidation process, and thesecond gate oxide layer 260 is thermally grown on the sidewall 232 andin the substrate 210. Therefore, the second gate oxide layer 260 has asidewall 262, and the sidewall 262 is exposed from the first trench 220.It should be noticed that a portion of the top surface 244 is in contactwith the second gate oxide layer 260, and a portion of the top surface244 is still exposed. In some embodiments, a part of the bottom portionof the second gate oxide layer 260 is like a bird's beak shape which isadjacent to the bottom portion 250 b of sacrificial layer 250. In someembodiments, the material of the second gate oxide layer 260 may besilicon oxide, such as tetraethylorthosilicate (TEOS), silicon dioxide(SiO₂), or silicon oxynitride (SiON), or combinations thereof, but notlimited thereto. In some embodiments, the material of the second gateoxide layer 260 is the same as the material of the first gate oxidelayer 240.

Please refer to FIG. 2F and FIG. 2G, the bottom portion 250 b of thesacrificial layer 250 is removed to form a second trench 270 (theoperation 160 of FIG. 1 ). Specifically, an inner surface 280 of thebottom portion 240 b of the first gate oxide layer 240 is exposed fromthe second trench 270. More specifically, the second trench 270 has thesidewall 262 of the second gate oxide layer 260 and the inner surface280 of the second trench 270. In some embodiments, the bottom portion250 b of the sacrificial layer 250 is removed by an etching process,such as wet etching, but not limited thereto.

Please refer to FIG. 2G and FIG. 2H, a gate layer 290 is formed in thebottom 272 of the second trench 270 (the operation 170 of FIG. 1 ). Insome embodiments, forming the gate layer 290 in the bottom 272 of thesecond trench includes the following operations. Specifically, a gatelayer is formed on the bottom portion 240 b of first gate oxide layer240 and the second gate oxide layer 260, and then an etch back processis performed to remove a portion of the gate layer to form the gatelayer 290. More specifically, in some embodiments, the gate layer 290has a top surface 292 substantially coplanar with the top surface 244 ofthe bottom portion 240 b of the first gate oxide layer 240. In someembodiments, the material of the gate layer 290 may be a conductivematerial, and the conductive material can be selected from the group ofamorphous silicon, polycrystalline silicon, polycrystalline silicongermanium, metal nitride, metal silicide, metal oxide and other suitablemetal. In some embodiments, the gate layer 290 is formed by depositionmethods such as chemical vapor deposition (CVD), atomic layer deposition(ALD), physical vapor deposition (PVD), or other suitable depositionprocesses.

Please refer to FIG. 2H and FIG. 2I, a dielectric layer 294 is formed onthe gate layer 290 in the second trench 270. Specifically, thedielectric layer 294 covers the top surface 292 of the gate layer 290and the sidewall 262 of the second gate oxide layer 260. In someembodiments, the dielectric layer 294 also covers a top surface 264 ofthe second gate oxide layer 260. In some embodiments, the dielectriclayer 294 includes silicon dioxide, silicon nitride, siliconoxy-nitride, tetraethoxysilane (TEOS) oxide, a low-k material and/or acombination thereof or other suitable dielectric material. In someembodiments, the dielectric layer 294 is formed by deposition methodssuch as chemical vapor deposition (CVD), atomic layer deposition (ALD),physical vapor deposition (PVD), or other suitable deposition processes.

Please refer to FIG. 2I and FIG. 2J, a source/drain region 296 is formedin the substrate 210 and adjacent to the gate layer 290. In someembodiments, a planarization process, such as chemical mechanicalpolishing (CMP), is performed before forming the source/drain region296. In detail, a top surface 295 of the dielectric layer 294 and a topsurface 261 of the second gate oxide layer 260 are exposed by performingthe planarization process. In some embodiments, the source/drain region296 is formed by performing an ion implantation process, but not limitedthereto. Specifically, the source/drain region 296 has a doping depth D1in the substrate 210. In some embodiments, the material of thesource/drain region 296 includes a doped semiconductor material such assilicon or germanium, a compound-doped semiconductor material such asgallium arsenide, indium arsenide, indium phosphide, or silicon carbide,or other suitable one or more of the materials. It is to be understoodthat the doping depth D1 and a doping concentration of the source/drainregion 296 can be adjusted.

Please refer to FIG. 2K. FIG. 2K is a cross-sectional view of a recessedaccess device 2000 in accordance with some embodiments of the presentdisclosure. The recessed access device 2000 includes a substrate 210, atrench 2020, a gate oxide layer 2010, a gate layer 290, and asource/drain region 296. The trench 2020 is in the substrate 210. Thetrench 2020 includes an upper portion 2022 and a lower portion 2024.Specifically, the upper portion 2022 has a substantially verticalsidewall 2030 and a first curved sidewall 2040 extending from a bottomedge 2050 of the substantially vertical sidewall 2030 to a top edge 2060of the lower portion 2024. The first curved sidewall 2040 of the upperportion 2022 and a sidewall 2070 of the lower portion 2024 of the trench2020 form an obtuse angle θ₁. More specifically, a maximum width W1 ofthe upper portion 2022 of the trench 2020 is greater than a maximumwidth W2 of the lower portion 2024 of the trench 2020.

Still refer to FIG. 2K. The gate oxide layer 2010 is disposed in thetrench 2020. Specifically, the gate oxide layer 2010 has a first portion2010 a on the substantially vertical sidewall 2030, a second portion2010 b on the sidewall 2070 of the lower portion 2024, and a connectionportion 2010 c on the first curved sidewall 2040 and between the firstportion 2010 a and the second portion 2010 b. A maximum thickness T_(C1)of the connection portion 2010 c is greater than a first thickness T₁₁of the first portion 2010 a and a second thickness T₁₂ of the secondportion 2010 b of the gate oxide layer 2010. The gate layer 290 isdisposed in the bottom 272 of the trench 2020 to cover the secondportion 2010 b and the connection portion 2010 c of the gate oxide layer2010. The source/drain region 296 is disposed in the substrate 210 andadjacent to a portion of the gate layer 290. It should be noticed thatthe connection portion 2010 c of the gate oxide layer 2010 has themaximum thickness T_(C1) greater than the first thickness T₁₁ and thesecond thickness T₁₂. Therefore, the present disclosure can improve theGIDL problem and would not affect the ability to control the gate layer290 of the recessed access device 2000.

The further detail of the recessed access device 2000 in FIG. 2K will bedescribed below. In some embodiments, the first thickness T₁₁ of thefirst portion 2010 a of the gate oxide layer 2010 is different from thesecond thickness T₁₂ of the second portion 2010 b of the gate oxidelayer 2010. For example, the first thickness T₁₁ is greater than thesecond thickness T₁₂, or the second thickness T₁₂ is greater than thefirst thickness T₁₁. In some embodiments, the first thickness T₁₁ of thefirst portion 2010 a of the gate oxide layer 2010 is substantially thesame as the second thickness T₁₂ of the second portion 2010 b of thegate oxide layer 2010. In some embodiments, the connection portion 2010c of the gate oxide layer 2010 has a top surface 244 substantiallycoplanar with a top surface 292 of the gate layer 290. In someembodiments, the source/drain region 296 is adjacent to the connectionportion 2010 c, the first portion 2010 a, and a portion of the secondportion 2010 b of the gate oxide layer 2010. In some embodiments, therecessed access device 2000 includes the dielectric layer 294 disposedon the gate layer 290.

FIG. 3 is a flowchart of a method of manufacturing a recessed accessdevice in accordance with some embodiments of the present disclosure.Operation 310, a first trench is formed in a substrate. The methodcontinues with operation 320 in which a sacrificial layer is formed in abottom of the first trench, in which a sidewall of the first trenchabove the sacrificial layer is exposed from the first trench. Operation330, the sidewall of the first trench is oxidized to form a first gateoxide layer. The method continues with operation 340 in which the firstgate oxide layer and the sacrificial layer are removed to form a secondtrench. Operation 350, an inner surface of the second trench is oxidizedto form a second gate oxide layer within the substrate. The methodcontinues with operation 360 in which a gate layer is formed in a bottomof the second trench. Operation 370, a source/drain region is formed inthe substrate and adjacent to the gate layer.

FIG. 4A to FIG. 4H are cross-sectional views of a method ofmanufacturing a recessed access device 4000 at various stages inaccordance with some embodiments of the present disclosure.

Please refer to FIG. 4A, a first trench 420 is formed in a substrate 410(the operation 310 in FIG. 3 ). The material of the substrate 410 can bethe same as the substrate 210 as mentioned in FIG. 2A, and the detailsthereof are not repeatedly described.

Please refer to FIG. 4A and FIG. 4B, a sacrificial layer 440 is formedin a bottom 434 of the first trench 420, in which the sidewall 432 ofthe first trench 420 above the sacrificial layer 440 is exposed from thefirst trench 420 (the operation 320 in FIG. 3 ). In some embodiments,forming the sacrificial layer 440 in the bottom 434 of the first trench420 includes the following operations. A sacrificial layer is formed inthe first trench 420, and then an etch back process is performed toremove a portion of the sacrificial layer to form the sacrificial layer440. After the etch back process is performed, the sacrificial layer 440is formed in the bottom 434 of the first trench 420, and the sidewall432 of the first trench 420 is exposed, as shown in FIG. 4B. Thematerial of the sacrificial layer 440 and the forming method thereof canbe the same as the material and the forming method of sacrificial layer250 as mentioned in FIG. 2C, and the details thereof are not repeatedlydescribed.

Please refer to FIG. 4B and FIG. 4C, the sidewall 432 of the firsttrench 420 is oxidized to form a first gate oxide layer 450 (theoperation 330 in FIG. 3 ). Specifically, the first gate oxide layer 450is formed by performing a thermal oxidation process, and the first gateoxide layer 450 is thermally grown on the sidewall 432 of the firsttrench 420 and in the substrate 410. More specifically, the first gateoxide layer 450 has a sidewall 452 and a sidewall 454, in which thesidewall 452 of the first gate oxide layer 450 is exposed from the firsttrench 420, and the sidewall 454 of the first gate oxide layer 450 is incontact with the substrate 410. In detail, a bottom edge of the sidewall454 has a curved sidewall 456, as shown in FIG. 4C. It should beunderstood that a top edge 442 of the sacrificial layer 440 is embeddedinto a bottom portion of the first gate oxide layer 450, after thesidewall 432 of the first trench 420 is oxidized. The material of thefirst gate oxide layer 450 may be silicon oxide, such astetraethylorthosilicate (TEOS), silicon dioxide (SiO₂), or siliconoxynitride (SiON), or combinations thereof, but not limited thereto.

Please refer to FIG. 4C and FIG. 4D, the first gate oxide layer 450 andthe sacrificial layer 440 are removed to form a second trench 460 (theoperation 340 in FIG. 3 ). In some embodiments, removing the first gateoxide layer 450 and the sacrificial layer 440 includes two etchingprocesses, and the etchants in such two etching process are different.

Please refer to FIG. 4D and FIG. 4E, an inner surface 470 of the secondtrench 460 is oxidized to form a second gate oxide layer 480 within thesubstrate 410 (the operation 350 in FIG. 3 ). Specifically, the secondgate oxide layer 480 is formed by performing a thermal oxidationprocess, and the second gate oxide layer 480 is thermally grown on theinner surface 470 and in the substrate 410. Therefore, an inner surface482 of the second gate oxide layer 480 is formed and exposed from thesecond trench 460. In detail, the second gate oxide layer 480 includes afirst portion 480 a, a second portion 480 b, and a connection portion480 c, in which the first portion 480 a is formed on the sidewall 454,the second portion 480 b is formed in the bottom 434, and the connectionportion 480 c is formed on the curved sidewall 456.

Please refer to FIG. 4E and FIG. 4F, a gate layer 490 is formed in abottom 462 of the second trench 460 (the operation 360 in FIG. 3 ). Insome embodiments, forming the gate layer 490 in the bottom 462 of thesecond trench 460 includes the following operations. A gate layer isformed on the first portion 480 a, the second portion 480 b, and theconnection portion 480 c of the second gate oxide layer 480. Then, anetch back process is performed to remove a portion of the gate layer,thereby exposing a sidewall 481 of the first portion 480 a of the secondgate oxide layer 480. After the etch back process is performed, the gatelayer 490 is formed in the second trench 460. The material of the gatelayer 490 and the forming method thereof can be the same as the materialand the forming method of the gate layer 260 as mentioned in FIG. 2H,and the details thereof are not repeatedly described.

Please refer to FIG. 4F and FIG. 4G, a dielectric layer 492 is formed onthe gate layer 490 in the second trench 460. Specifically, thedielectric layer 492 covers a top surface 491 of the gate layer 490 andthe sidewall 481 of the first portion 480 a of the second gate oxidelayer 480. In some embodiments, the dielectric layer 492 also covers atop surface 483 of the second gate oxide layer 480. The material of thedielectric layer 492 and the forming method thereof can be the same asthe material and the forming method of the dielectric layer 294 asmentioned in FIG. 2I, and the details thereof are not repeatedlydescribed.

Please refer to FIG. 4G and FIG. 4H, a source/drain region 494 is formedin the substrate 410 and adjacent to the gate layer 490 (the operation370 in FIG. 3 ). In some embodiments, a planarization process, such asCMP, is performed before forming the source/drain region 494. In detail,a top surface 493 of the dielectric layer 492 and the top surface 483 ofthe second gate oxide layer 480 are exposed by performing theplanarization process. In some embodiments, the source/drain region 494is formed by performing an ion implantation process, but not limitedthereto. Specifically, the source/drain region 494 has a doping depth D2in the substrate 410. The material of the source/drain region 494 can bethe same as that of the source/drain region 296 as mentioned in FIG. 2J.It is to be understood that the doping depth D2 and a dopingconcentration of the source/drain region 494 can be adjusted.

Please refer to FIG. 4H again. FIG. 4H is a cross-sectional view of arecessed access device 4000 in accordance with some embodiments of thepresent disclosure. The recessed access device 4000 includes a substrate410, a trench 4020, a gate oxide layer 480, a gate layer 490, and asource/drain region 494. The trench 4020 is in the substrate 410. Thetrench 4020 includes an upper portion 4022 and a lower portion 4024.Specifically, the upper portion 4022 of the trench 4020 has asubstantially vertical sidewall 4030 and a first curved sidewall 4040extending from a bottom edge 4050 of the substantially vertical sidewall4030 to a top edge 4060 of a sidewall 4070 of the lower portion 4024 ofthe trench 4020. The first curved sidewall 4040 of the upper portion4022 of the trench 4020 and the sidewall 4070 of lower portion 4024 ofthe trench 4020 form an obtuse angle θ₂. More specifically, a maximumwidth W3 of the upper portion 4022 of the trench 4020 is greater than amaximum width W4 of the lower portion 4024 the trench 4020.

Still refer to FIG. 4H. The gate oxide layer 480 is disposed in thetrench 4020. The gate oxide layer 480 has a first portion 480 a on thesubstantially vertical sidewall 4030 of the upper portion 4022 of thetrench 4020, a second portion 480 b on the sidewall 4070 of the lowerportion 4024 of the trench 4020, and a connection portion 480 c on thefirst curved sidewall 4040 of the upper portion 4022 of the trench 4020.The connection portion 480 c is disposed between the first portion 480 aand the second portion 480 b of the gate oxide layer 480. A maximumthickness T_(C2) of the connection portion 480 c of the gate oxide layer480 is greater than a first thickness T₂₁ of the first portion 480 a ofthe gate oxide layer 480. The maximum thickness T_(C2) of the connectionportion 480 c of the gate oxide layer 480 is greater than a secondthickness T₂₂ of the second portion 480 b of the gate oxide layer 480.The gate layer 490 is disposed in the bottom 462 of the trench 4020 tocover the second portion 480 b and the connection portion 480 c of thegate oxide layer 480. The source/drain region 496 is disposed in thesubstrate 410 and adjacent to the gate layer 490. It should be noticedthat the connection portion 480 c of the gate oxide layer 480 has themaximum thickness T_(C2) greater than the first thickness T₂₁ and thesecond thickness T₂₂. Therefore, the present disclosure can improve theGIDL problem and would not affect the ability to control the gate layer490 of the recessed access device 4000.

The further detail of the recessed access device 4000 in FIG. 4H will bedescribed below. In some embodiments, the first thickness T₂₁ of thefirst portion 480 a of the gate oxide layer 480 is substantially thesame as the second thickness T₂₂ of the second portion 480 b of the gateoxide layer 480. In some embodiments, the connection portion 480 c ofthe gate oxide layer 480 includes a second curved sidewall 4090 facingtoward the gate layer 490, and the second curved sidewall 4090 extendsfrom a top edge 4092 of the gate layer 490 to a top edge 4094 of thesecond portion 480 b of the gate oxide layer 480. In some embodiments,the source/drain region 494 is adjacent to the connection portion 480 c,the first portion 480 a, and a portion of the second portion 480 b ofthe gate oxide layer 480. In some embodiments, the recessed accessdevice 4000 includes the dielectric layer 492 disposed on the gate layer490.

The present disclosure can improve the GIDL problem occurred in theoverlap region between a gate layer and a source/drain region in thesemiconductor device. The gate oxide layer of the recessed access devicehas the connection portion with the thickness greater than thethicknesses of other portions of the gate oxide layer. The greaterthickness of the connection portion of the gate oxide layer would notaffect the ability to control the gate layer of the recessed accessdevice. This is because the size of the gate layer would not be reduced;therefore, the ability to control the gate would not be affected.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A recessed access device, comprising: a substratehaving a trench embedded therein, wherein the trench comprises an upperportion and a lower portion, the upper portion has a substantiallyvertical sidewall and a first curved sidewall extending from a bottomedge of the substantially vertical sidewall to a top edge of the lowerportion, and the first curved sidewall of the upper portion and asidewall of the lower portion form an obtuse angle; a gate oxide layerdisposed in the trench, wherein the gate oxide layer has a first portionon the substantially vertical sidewall, a second portion on the sidewallof the lower portion, and a connection portion on the first curvedsidewall and between the first portion and the second portion, wherein amaximum thickness of the connection portion is greater than a firstthickness of the first portion and a second thickness of the secondportion; a gate layer disposed in a bottom of the trench to cover thesecond portion and the connection portion of the gate oxide layer; and asource/drain region disposed in the substrate and adjacent to the gatelayer.
 2. The recessed access device of claim 1, wherein the firstthickness of the first portion is substantially the same as the secondthickness of the second portion.
 3. The recessed access device of claim1, wherein the first thickness of the first portion is different fromthe second thickness of the second portion.
 4. The recessed accessdevice of claim 1, wherein the connection portion has a top surfacesubstantially coplanar with a top surface of the gate layer.
 5. Therecessed access device of claim 1, wherein the connection portioncomprises a second curved sidewall facing toward the gate layer, and thesecond curved sidewall extends from a top edge of the gate layer to atop edge of the second portion of the gate oxide layer.
 6. The recessedaccess device of claim 1, wherein the source/drain region is adjacent tothe connection portion, the first portion, and a portion of the secondportion of the gate oxide layer.
 7. The recessed access device of claim1, further comprising a dielectric layer disposed on the gate layer.